1. Field of the Invention
This invention relates to a novel process for producing 4-methyl-1-pentene by the dimerization of propylene.
4-Methyl-1-pentene polymers are excellent in transparency, heat resistance, mechanical and electrical properties and chemical resistance. It is also a compound capable of exhibiting particularly excellent properties as the comcnomer for improving various properties of polyolefins, such as transparency, environmental stress crack resistance and like properties.
2. Description of the Prior Art
It has been known that 4-methyl-1-pentene can be obtained by dimerizing propylene in the presence of an alkali metal such as sodium or potassium [reported for example by A. W. Shaw et al in J. Org. Chem., 30, 3286(1965)].
It has also been known to use for example an alcohol (Japanese Patent Publication No. 8701/1963), phenol (Japanese Patent Publication No. 19622/1964) or tertiary amine (Japanese Patent Laid-open No. 93303/1974) when conducting the dimerization reaction of propylene in the presence of an alkali metal.
It has also been known that 4-methyl-1-pentene can be obtained by dimerizing propylene in the presence of an alkali metal supported on a carrier. In this case, graphite, potassium carbonate, alkali metal silicates, alkali metal halides, magnesium sulfate, talc or the like may be used as the carrier.
However, the above-mentioned and the other known processes are accompanied by such disadvantages that the yield of propylene dimer and the selectivity of 4-methyl-1-pentene are both relatively low and besides the desired 4-methyl-1-pentene, by-products, such as cis- and trans-4-methyl-2-pentene, 2-methyl-1-pentene, 2-methyl-2-pentene, 1-hexene, cis- and trans-2-hexene and cis- and trans-3-hexene are produced in large amounts. The boiling points of these isomers are close to the desired product, 4-methyl-1-pentene, and therefore, a high degree of fractionation is required to obtain 4-methyl-1-pentene in sufficiently high purity. This becomes another disadvantage because of high cost for purification.
Further, catalysts used in the known processes will take a long induction periods, i.e., they require a great deal of time until their full activities are exhibited. In other words, they require a long period of time until the reaction proceeds stationarily. Hence, many of these known processes were inferior from the viewpoint of economy and stable operation. Moreover, many of these known dimerizing catalysts have not only a capability of catalytic dimerization but also a capability of catalytic polymerization. Polymerization reactions is allowed to proceed along with the dimerization reaction, and the polymers so produced deposit on the surfaces of such catalyst, resulting in the gradual loss of their activities. It is often observed that the selectivity tends to decrease as the activity decreases, particularly when such a catalyst is used. A catalyst which has lost its activity in the above manner solidified with resinous polymers in the reactor. However, it internally contains still sufficiently highly active portions. When such a spent catalyst is drawn out for replacement with a fresh catalyst, there is thus a potential danger that it may burn or induce fire due to contact with oxygen, water or the like in the air. Thus, such catalysts are accompanied by the further disadvantage that their handling is inconvenient.
In the conventional catalysts used for the production of 4-methyl-1-pentene, the amounts of sodium or potassium supported on carriers were below 5 wt. %, usually within the range of 1-3 wt. % or so because the carriers have relatively small pore volume. When more than 5 wt. % of sodium or potassium is attempted to be loaded on the carrier, these alkali metals deposited in the form of mud on the surfaces of the carriers. Thus, the catalysts agglomerated into lumps, thereby not only making their industrial handling difficult but also significantly lowering their dimerization activities in many cases. In order to carry out the dimerization reaction with fixed-bed method, it is necessary to form each catalyst into pellets. It was however impossible to pelletize potassium carbonate which has been conventionally used as a carrier for 4-methyl-1-pentene catalysts, because it does not have caking property by itself. Thus, many of known catalysts were prepared by producing pellets using graphite or the like as a binder and then thus-produced pellets were treated by sodium or potassium. These pellets were however accompanied by such a disadvantage that their service life as catalysts became short due to their low mechanical strength which relates to their tendency of disintegration during their use.